This invention relates to a method of producing thin gauge cube-on-edge oriented silicon-iron sheet using a two-stage cold reduction. More particularly, the invention relates to a more economical method of producing thin gauge, using only two cold-reduction steps to provide a steel having magnetic properties of core loss and magnetic permeability comparable or better than any conventional three-stage process.
In the manufacture of grain-oriented silicon steel, it is known that if improved secondary recrystallization texture is achieved, the magnetic properties, particularly permeability and core loss, will be correspondingly improved. The Gauss texture (110)[001], in accordance with Miller's Indices, refers to the body-centered cubes making up the grains or crystals being oriented in the cube-on-edge position. The texture or grain orientation of this type refers to the cube edges being parallel to the rolling direction and in the plane of rolling, and the cube face diagonals being perpendicular to the rolling direction and in the rolling plane. As is well known, steel having this orientation is characterized by a relatively high permeability in the rolling direction and a relatively low permeability in a direction at right angles thereto.
Grain-oriented silicon steel is useful as cores for distribution and power transformers and generators and, thus, it is important that the steel be characterized by good magnetic permeability and core loss values. It is known that the core loss is made up of two main components, that due to the hysteresis effect, and that due to eddy currents. The magnitude of the eddy currents is also limited by the resistance of the path through which they flow. The resistance of the core material is determined by the resistivity of the material and its thickness or cross-sectional area. Consequently, it is desirable that technically important magnetic materials have a high resistivity and be produced in thin sheets in order that eddy current losses can be kept to a minimum.
In recognition of the advantages of providing a thin gauge silicon-iron sheet, numerous attempts have been made to produce such thin gauge material of less than 0.01 inch (.254 mm) U.S. Pat. No. 3,586,545, issued June 22, 1971, discloses a method of producing cube-on-edge oriented silicon-iron sheet stock 0.005 to 0.010 inch (0.127 to 0.254 mm) thick using a triple cold reduction of the silicon-iron hot band. Each cold reduction is followed by an annealing operation. The method of that patent further requires that the first and third cold reduction be about a 50% thickness reduction.
U.S. Pat. No. 3,632,456, issued Jan. 4, 1972, discloses a process for producing thin gauge electromagnetic steel sheet using a two-stage cold reduction process. The process of that patent, however, requires a specific composition of the steel such that it contains 0.010 to 0.065% acid soluble aluminum and requires the step of annealing the hot-rolled sheet to cause AlN to precipitate in the steel sheet prior to the two cold rolling steps. The final cold rolling is carried out at a reduction rate of 70 to 95% thickness reduction to a final gauge of 0.35 to 0.05 mm (0.014 to 0.002 inch).
Neither of the patents teach or suggest the two-stage cold reduction process of the present invention for producing conventional grain-oriented silicon steel in thin gauges of less than 0.0085 inch (0.216 mm). Though conventional two-stage cold rolling reduction processes are known for producing standard gauge 10-14 mil grain-oriented silicon steel, it is desirable to provide an economical process for producing thin gauge silicon-iron sheet.
What is needed is a process to produce thin gauge conventional grain-oriented silicon steel with comparable or better magnetic properties, particularly core loss and magnetic permeability, than can be available with more conventional three-stage cold rolling processes. The improved process should consistently produce thin gauge silicon-iron sheet having a combination of good magnetic permeability and core loss, such as is suitable for distribution transformers at low frequencies and relatively high inductions. The improved process should also be sufficiently economical to allow production of a thin gauge product which is competitive with commercial thicker gauge products presently on the market. It is also desirable to provide a thin gauge silicon-iron sheet of nominally 7 mils characterized by a core loss of less than 0.720 WPP at 60 Hertz at 17 KG, and a magnetic permeability of at least 1820 (G/O.sub.e) at 10 oersteds.